The effect of varying molecular weight distribution on the properties of binders

Work is reported on the effect of variation of molecular weight distribution on the properties of binder stocks. This work used three prepolymers (polypropylene glycol 1000, 2000, and 4000 and polypropylene glycol 400, 1000, and 2000) of different molecular weight and known molecular weight distribution. The prepolymer of the middle molecular weight was employed as standard, and stocks were prepared from it. To this prepolymer was added some of the high and low molecular weight prepolymers in proportions such that the crosslink density of stocks made from the mixture was the same as that of stocks made from the middle molecular weight prepolymer alone. Two series were prepared with different crosslinking agents. The proportions of high and low molecular weight prepolymer were successively increased until finally none of the middle molecular weight remained. Heterogeneity indexes of the glycols and their mixtures were measured by gel permeation chromatography. Elongation at break, modulus of elasticity, and extent of solvent swelling were determined on the crosslinked stocks. Properties which vary with crosslink density were found to show decreasing values with increasing heterogeneity index even though the stocks were formulated to a constant crosslink density. Narrow-distribution stocks reach the maximum degree of cure faster than the broader-distribution stocks. Infrared and thermal analysis confirm that monodisperse polymer has a greater extent of reaction than heterodisperse polymer. Results showed that the variation in elongation at break to be expected because of lot-to-lot variations in heterogeneity index is probably not greater than the experimental variation in the elongation test below a heterogeneity index of 1.5. Above 1.5, however, if the effects observed are entirely due to variation in heterogeneity index, lot-to-lot variations in molecular weight distribution cannot be ignored. An explanation is presented based on the varying ability of prepolymer molecules of different size to diffuse through uniform mesh openings resulting in lower final extents of reaction for broad distribution material.     

Free volume in PEP-silica nanocomposites with varying molecular weight

The influence of confinement in polymer-nanocomposites on free volume and glass transition of the polymer chains was studied. The molecular weight (Mw) of poly(ethylene-alt-propylene) (PEP) was varied from 3k to 200k and so the end-to end distance of the chains at fixed diameter (∼15 nm) and concentration (15%) of the silica nanoparticles increased. Thus the topological confinement increases with increasing Mw. Using hydrophobic PEP and particles functionalized with short organic molecules, we can rule out contributions of permanent adsorption of the chains. DSC showed no change in glass transition temperature. The decrease in the specific heat capacity could be explained by a simple mixing rule. By positron annihilation lifetime spectroscopy, taking properly into account contributions of the silica particles, we rule out an influence of the geometrical confinement on the free volume in the PEP nanocomposites studied here.     

High speed molecular weight distribution

It is now possible to do molecular weight distribution of polymeric material almost as fast as you can put them in solution. μSYRAGEL® has opened a new era in the determination of molecular weight distribution of polymers. With operational efficiencies of 800 plates/min., one can obtain a molecular weight distribution in 15 mins. Combining three of the newest advances in liquid chromatography—solvent delivery, septumless injection, and μSYRAGEL—now makes it possible to use molecular weight distribution as a quality control tool in polymerization and fabrication. The reproducibility of the molecular weight distribution is now good enough that overlays can be done from the raw data. The chromatograms can be used right off the recorder. The reproducibility of the weight average and number average molecular weight is 2–3 percent.     

The molecular weight distribution of oligomers

It is shown that the relation $\text{P}{}_{\mathrm{w}}\text{P}{}_{\mathrm{n}}\text{= 2}$ or the nonuniformity U = 1 is only a limiting case for high degrees of polymerization, is no more valid for oligomers. Two new relations are derived, one including and one excluding monomers. For the oligomer part of cationically produced polystyrenes the second relation is in good agreement with experiments.     

Effect of molecular weight on crystallization isotherms of high molecular weight poly(ethylene oxide) fractions

Dilatometric crystallization isotherms have been determined for a set of poly(ethylene oxide) fractions ranging in molecular weight from 2 × 10⁴ to 1.6 × 10⁶. For a given fraction the isotherms obtained for different crystallization temperatures can be superimposed over most of the crystallization. For a given crystallization temperature the degree of crystallinity obtained in the primary stage of the crystallization varies greatly with molecular weight, and superimposition of the isotherms is not possible. Secondary crystallization processes are pronounced when the molecular weight ($\text{M}\text{?}{}_{\mathrm{v}}$) exceeds 10⁵.     

Influence of thermal treatments,molecular weight,and molecular weight distribution on the crystallization of β‐isotactic polypropylene

Six samples of isotactic polypropylene were examined to study the influence of the thermal treatments and the molecular weights and their distribution on the β‐crystallization of the polymer. The highest amount of the β‐phase was obtained by isothermal crystallization and in correspondence of high average molecular weights and wide molecular weight distributions. Small‐angle X‐ray scattering pointed out that a partial β‐crystallization seems not to influence the lamellar morphology parameters. Differential scanning calorimetry measurements, at different heating rates, allowed us to confirm that the multiple melting endotherms behavior of the β‐phase is to be correlated to a melting–recrystallization–melting mechanism. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1008–1012, 2004     

Molecular weight characterization of polybutadiene rubber with high molecular weight and broad molecular weight distribution

Following the molecular weight characterization of two polybutadiene samples, it was found that M _w from gel permeation chromatography with universal calibration and light scattering were in agreement, but M _n by gel permeation chromatography was less than M _n from membrane osmometry. A more detailed analysis revealed that the high molecular weight and broad molecular weight distribution of the two samples forced two corrections to the membrane osmometry results for (a) diffusional layer effects caused by high solution viscosities, and (b) solute permeability of the membrane. In the latter effect, the high viscosities of the solutions prevented actual diffusion through the membrane, but “reflection” of these species as defined by the Staverman coefficient prevented an accurate M _n determination. After making these corrections, it was found that M _n from membrane osmometry using a very tight membrane was in very good agreement with M _n from gel permeation chromatography. A method is demonstrated for obtaining M _n from a combination of membrane osmometry and gel permeation chromatography, where membrane osmometry data from membranes of different porosities (after corrections for diffusional layers and membrane reflection) are used to verify the accuracy of the gel permeation chromatography data as representing the true molecular weight distribution, allowing the gel permeation chromatography data to be used to calculate M _n.     

Stirring-induced solution crystallization of ultra high molecular weight polyamide 6

Unstable solutions of ultrahigh molecular weight polyamide 6 have been prepared by adding a nonsolvent to the polymer solution. Crystallization of the polyamide from such a solution proceeds very slowly. It has been found, however, that vigorous stirring of the unstable solutions induces rapid fibrous crystallization of the polymer. The fiber mat consists of irregularly shaped fibers. A low temperature and a high stirring rate are among the conditions necessary to obtain a high yield of fibrous material. The fibers formed upon stirring have a higher molecular weight than the polyamide 6 molecules which remain in the solution. The melting point of the fibers depends on the speed of the paddle stirrer. The differential scanning calorimetry (DSC) thermogram reveals higher melting temperatures of the fibrous material if higher stirring rates have been applied.     

聚丙烯熔体流动指数与分子量及其分布的关系

研究了聚丙烯(PP)的熔体流动指数(MI)与聚合物不同分子量之间的关联性,对于分子量分布较窄的PP,数均分子量(Mn)、重均分子量(Mw)和粘均分子量(Mv)均能较好的关联;反之,MI与Mn关联性下降,而MI与Mn和Mv的关联性仍很好,尤其是MI与Mv的关联性受分子量分布的影响很小;MI与Z均分子量的关联性很差。同时．确定了MI与各种分子量之间的关联式,该式用于本体PP工艺反应器内氢气浓度的计算和MI的预测,与实验测量结果吻合良好。     

凝胶色谱法在聚丙烯分子量及分子量分布测定中的应用

根据聚丙烯在凝胶色谱柱上的淋洗特点,确定了测定其分子量及分子量分布试验条件;采用示差和黏度双检测凝胶色谱系统,利用普适校正方法,不需要Mark常数K、α值,可直接测定聚丙烯分子量分布并给出分子量分布曲线;采用统计方法计算数均分子量、重均分子量、黏均分子量,可以为课题研究提供一定的依据和指导。     

A cooperative molecular weight distribution test

The development of gel permeation chromatography (GPC) has provided a convenient tool for the rapid determination of molecular weight distribution. The question has arisen as to the suitability of the method for specification purposes. The present work, suggested by the Naval Air Systems Command, represents an attempt to assess the precision of the method through a series of tests carried out by a number of laboratories using identical procedures on the same samples. Ten laboratories agreed to take part. Naval Ordnance Station, Indian Head, worked out standard conditions for operation of the chromatograph, for calibration of the columns, and for analysis of the GPC curves. Two samples of polystyrene were used by the various organizations for calibration of their instruments. Number-average molecular weight, heterogeneity index, and cumulative molecular weight distribution curves were determined on four samples of carboxyl-terminated polybutadiene (CTPB) and two samples of hydroxyl-terminated polybutadiene (HTPB), all unidentified except by letter code. All laboratories used identical directions for setting up CTPB and HTPB calibration curves which were based on curves determined from vapor-pressure osmometer molecular weights and GPC count numbers of fractionated material. Variation among the different laboratories was 0.15 in heterogeneity index, and a maximum of 1200 in molecular weight provided one aberrant set of values was eliminated. The six samples had heterogeneity indices from 1.15 to 1.54, while molecular weight varied from approximately 3000 to 6000. The average coefficient of variation of the molecular weight values was 6.2 ± 0.7%, which is quite acceptable. Variation in heterogeneity index was too great for specification purposes when considered among the different laboratories, but may be sufficiently good when measured by any one laboratory.     

Effects of molecular weight and molecular weight distribution on creep properties of polypropylene homopolymer

The molecular weights of the industrial-grade isotactic polypropylene (i-PP) homopolymers samples were determined by the melt-state rheological method and effects of molecular weight and molecular weight distribution on solid and melt state creep properties were investigated in detail. The melt-state creep test results showed that the creep resistance of the samples increased by M_w due to the increased chain entanglements, while variations in the polydispersity index (PDI) values did not cause a considerable change in the creep strain values. Moreover, the solid-state creep test results showed that creep strain values increased by M_w and PDI due to the decreasing amount of crystalline structure in the polymer. The results also showed that the amount of crystalline segment was more effective than chain entanglements that were caused by long polymer chains on the creep resistance of the polymers. Modeling the solid-state viscoelastic structure of the samples by the Burger model revealed that the weight of the viscous strain in the total creep strain increased with M_w and PDI, which meant that the differences in the creep strain values of the samples would be more pronounced at extended periods of time.     

The preparation of ultra narrow molecular weight distribution poly(ethylene glycol)s by fractional crystallization from solution

The preparation of very narrow molecular weight distribution poly(ethylene glycol) (PEG) was investigated by fractional crystallization from solution. Fractionation experiments were conducted from the solutions of 1500, 2000, and 3000 molecular weight PEG in n‐butanol/n‐heptane mixtures. Size Exclusion Chromatography and Differential Scanning Calorimetric studies were performed for the characterization of fractions prepared at different crystallization temperatures and times. By suitable choice of these experimental parameters it was possible to obtain PEG fractions with M_w/M_n ≤ 1.05. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1999–2005, 2001     

The effect of molecular weight in the crystallization kinetics of glassy nylon 6

Crystallization of glassy nylon 6 has been investigated with differential scanning calorimetry and wide angle X-ray diffraction in order to determine the effect of average molecular weight. Samples were prepared by quenching molten films between platens: chilled with liquid nitrogen. These films had number average molecular weights ranging from 10,000 to 42,000 and had polydispersity indices ranging from 2.0 to 3.1. Crystallization kinetics at large undercoolings were measured in terms of the systematic dependence of conversion half time, Avrami exponent and heat of crystallization. Number average molecular weight was observed to influence crystallization rate at large undercoolings through its effect on glass transition temperature. At intermediate undercoolings, molecular weight affected crystallization rate through terms other than the glass transition temperature. In addition, a dependence on the degree of order of the macromolecules, both prior to and after crystallization, was observed.     

利用输出反馈方法实现聚合物分子量分布的控制

利用正交多项式组合神经网络对聚合反应分子量分布(MWD)进行建模,MWD被解构为若干矩值组成的矩向量函数,由此二元MWD控制问题可被转换为一元分布矩控制问题。以矩向量为直接被控对象,通过对矩的控制实现MWD的跟踪。为便于求解这类非仿射非线性多变量系统的控制策略,提出了改进型非线性自回归正交多项式网络结构,建立模型输出与U(k)之间的线性映射关系;针对高维被控矩向量,证明了矩向量中独立变量与分布函数参变量间的数量对等关系,给出了矩向量降维准则,将高维输出控制转化为低维问题。基于改进的神经网络模型,利用输出反馈方法对MWD矩向量进行控制,实现了对MWD的形状跟踪,仿真实验证明了方法的有效性。所提出的方法为非线性多变量系统的建模控制问题提供了新思路。     

Influence of molecular weight and molecular weight distribution on the tensile properties of amorphous polymers

Tensile property data for polystyrene samples of varying polydispersity are correlated with various parametric measures of molecular weight. Traditional measures of molecular weight, such as M?_n, M?_w, and M?_z, are shown to be unable to account for the variation of tensile properties with molecular weight. However, a new molecular weight parameter, termed the failure property parameter, is able to provide a single relationship between tensile strength and the parameter for both the broad and narrow distribution polymers. The form of this parameter is consistent with its having origins in the view that it is the entanglement network in an amorphous polymer that provides the observed strength properties. Specifically for polystyrene, the failure property parameter results indicate that material below 60,000 molecular weight does not contribute to polymer strength. Although the results of this investigation are specifically for polystyrene, the arguments used to develop the failure property parameter are not dependent on polymer chemical structure. Consequently, we believe that both the concepts and definition of this new parameter are applicable to all amorphous polymers.